Add a hardwired-blend-mode version of GrBlendFragmentProcessor::Make.

When `Make<BlendOp>(src, dst)` is used, the blend mode will always be
hard-coded into the shader, instead of using a uniform to apply a
generic Porter-Duff blend.

Change-Id: I59a05eea3417b1880cef63738c0116d19f01ee3e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/525641
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
8 files changed